Ultrasound devices typically include an ultrasonic transducer with scan/receive heads, a processing unit, and a display device. Ultrasonic transducers may be adapted for special applications such as, for example, transcutaneous applications, laparoscopic applications, transrectal applications, endo-cavity probes, vaginal probes, 2D and 3D phased array probes, and/or other probes or applications. Laparoscopic ultrasound devices typically include an ultrasonic transducer with a movable scan head, a handle, and an apparatus for controlling the orientation of the scan head. An image-guided ultrasound device makes use of sensor elements or position indicating elements placed into the head of the ultrasound transducer and a companion tracking device that can determine the position and/or orientation of sensor elements in the coordinate system of the tracking device. After calibrating the sensor elements to the scan plane of the ultrasonic transducer, the position and/or orientation of the scan plane can also be determined. Using registration techniques, position and orientation information regarding the scan plane of the ultrasound device enables merging and integration of live ultrasound images to preoperative scans (e.g., computerized tomography [CT] scans, magnetic resonance [MR] scans, or other images). Information regarding registration of position/orientation data to image/scan data, production of merged or integrated images therefrom, and/or other information useful with the invention may be found in U.S. patent application Ser. No. 11/059,336 (U.S. Patent Publication No. 2005/0182319), entitled “Method and Apparatus for Registration, Verification, and Referencing of Internal Organs,” which is hereby incorporated by reference herein in its entirety.
Attachment of one or more electromagnetically tracked sensor elements to an ultrasound such as, for example, a laparoscopic ultrasound, may offer a number of advantages over the state of the art in trackable ultrasound devices. For example, the scan plane of an ultrasonic transducer can be tracked within the patient's body (as opposed to optical tracking, which is limited to line-of-sight applications) for the aforementioned purpose of superimposing ultrasound images to preoperative images. In addition, it would be advantageous to utilize devices and methods that do not require constant recalibration of the sensor elements with the scan plane in situations where the relative positions of the sensor elements and the ultrasound transducer are changed after initial calibration. By placing a sensor element adjacent to the transducer of an ultrasound device, the device is intrinsically more accurate because the “lever effect” caused when the sensor is located far from the transducer is minimized. The “lever effect” causes a small error in angular measurement by the sensor element to appear as a large error in the position of the transducer.